This invention relates to methods and equipment or systems for rinsing of semiconductor substrates such as partially processed wafers and similar materials. It is particularly useful for ring semiconductor substrates which have had one or more layers of metal (for example metal interconnects) deposited on them, such as during Back End of the Line (BEOL) processing in a typical semiconductor wafer fabrication sequence; however, certain aspects of the invention are also applicable to rinsing of semiconductor substrates during their manufacture, prior to deposition of metal on them, i.e. in Front End of the Line processing. In one aspect, the invention incorporates a method for preventing corrosion of said metal layers during the wet processing steps which often follow a metal etch in the BEOL process sequence.
Metallization systems used in modern integrated circuits have become increasingly complex due to the reliability concerns and yield losses associated with issues such as interdiffusion and electromigration. Solutions to these problems have involved the use of metal alloys and multilayer bar metal schemes. Although these approaches have been effective at solving some problems, they have also introduced a new set of problems centered on the difficulty in etching these multi-metal systems. Plasma etch processes and their associated chemistries have become more aggressive. The net result has been the formation of complex, insoluble etch residues inside the metal vias and on the metal sidewalls. The mixed nature of these residues—often composed of organic, inorganic, and metallic species—has required the use of aggressive wet chemical strippers. Very often these processes involve the use of highly alkaline media such as NMP (N-methylpyrrolidinone), and various other amine-containing compounds such as HDA (hydroxylamine). An unfortunate side effect associated with such wet processing is metal corrosion, where the process or rinse solution can act as the electrolyte in the corrosion processes. Corrosion can occur within a via, increasing its contact resistance or on a metal interconnect, increasing its sheet resistance.
One currently used solution to this metal corrosion problem in BEOL processing is the use of an intermediate solvent rinse, such as isopropanol prior to immersion in a final rinse with aqueous media such as Deionized(DI) water. The function of the intermediate rinse is to dilute and/or neutralize chemicals remaining on the wafer surfaces prior to subsequent rinsing and cleaning steps. However, there are serious drawbacks to the use of isopropanol as an intermediate rinsing step. The major concerns are safety issues associated with using such a flammable substance at the elevated temperatures typically used. Environmental concerns can preclude or limit its use in some locations. In addition, the use of isopropanol adds a process vessel, equipment and step to the process sequence, resulting in increased cost to the overall wet processing equipment and sequence. The net result is an increase in the wet bench footprint, that is, the floor space used, an increase in chemical usage and a reduction in wafer throughput.
It is highly desirable to minimize the number of processing steps and the amount and size of apparatus and equipment utilized in carrying them out. Such processing is generally conducted in clean rooms in which floor space is at a premium. Thus, the amount of floor space occupied by equipment is extremely critical to the overall cost and efficiency of the semiconductor fabrication process, particularly in light of today's economic conditions in which the prices of semiconductors and chips are highly competitive and tend to decrease.
It is an object of this invention therefore to provide a system for rinsing metallized semiconductor substrates with at most minimal additional corrosion during this step, which is relatively compact, inexpensive, efficient and easy to operate.
The prior art contains disclosures of the use of certain chemical agents in the rinsing of substrates, mainly in the front-end processing of wafers, that is, in the processing of wafers prior to the deposition of metallic components such as lines or films. This has included the addition of various agents to an aqueous (usually ultrapure or deionized water) front-end-of-the-line rinse step, where there are no metal layers on the substrate, and therefore metal corrosion is not a concern. Such techniques are described for example in U.S. Pat. No. 5,382,296 and European Patent Application 784,336, in which various materials are recommended for front-end treatment, including ozone, various inorganic acids, combinations of inorganic acids and other materials, and salts. Other known front-end rinsing techniques involve materials such as hydrogen peroxide (European Patent 560,324), combinations of amines with amino acids (European Patent Application 678,788), aqueous solutions combining hydrogen fluoride and hydrogen peroxide (European Patent Application 649,168), and combinations of amines and fatty acids (U.S. Pat. No. 5,102,777).
Vos, et al., in the paper “A Novel Environmentally-friendly Corrosion-Free Post-stripping Rinsing Procedure after Solvent Strip (1997 Symposium on VLSI technology, Kyoto, Japan) disclose the use of an acidic solution comprising diluted nitric acid for rinsing metallized aluminum wafers. These results are also reported in their European patent application 846,985. Carbon dioxide is disclosed in U.S. Pat. No. 5,175,124 for rinsing substrates in both front-end (i.e., non-metallized substrates) and back-end (i.e., metallized substrates) processing steps.